Luminance-enhancing polarising plate for an organic light-emitting element

ABSTRACT

There is provided a polarizer for organic light emitting diodes (OLED) having improved brightness. The polarizer, which comprises a linear polarizer and a ¼ retardation plate, comprises a reflective polarizer film disposed between the linear polarizer and the ¼ retardation plate and transmitting a polarized light horizontal to the transmission axis of the linear polarizer while reflecting a polarized light vertical to the transmission axis of the linear polarizer. The polarizer may be useful to highly improve the brightness of the OLED device when the polarizer is used in the OLED device.

This application is a 35 U.S.C. §371 National Stage entry ofInternational Application No. PCT/KR2009/002704, filed on May 22, 2009,and claims priority to Korean Application No. 10-2008-0047717, filed onMay 22, 2008 and Korean Application No. 10-2009-0044511, filed on May21, 2009, which are all hereby incorporated by reference in theirentireties.

TECHNICAL FIELD

The present invention relates to a polarizer for organic light emittingdiodes (OLED), and more particularly, to a polarizer that is developedto highly improve the brightness of the OLED.

BACKGROUND ART

An organic light emitting diode (OLED) refers to ‘a spontaneouslylight-emitting organic material’ that emits light by usingelectroluminescence where a fluorescent organic compound is allowed toemit light when an electric current is applied to the fluorescentorganic compound. This OLED may be driven at a relatively low voltageand manufactured with a small thickness, and has a wide viewing angleand a swift response time as well. Therefore, the OLED has advantages inthat, unlike the liquid crystal displays (LCD), it has an unchangedimage quality and no image sticking even when seen right by the side,and may realize its full coloration as well. Therefore, the OLED has ahigh potential as one of leading devices of the next-generation flatpanel display.

Such an OLED is generally formed by sequentially stacking an anode (ITOlayer), an electron injection layer, an electron transport layer, anemissive layer, a hole transport layer, a hole injecting layer and acathode on a transparent substrate. Here, electrons start to move when avoltage is applied to the OLED. In this case, in the cathode, theelectrons move to the emissive layer by the aid of the electrontransport layer. On the while, in the anode, holes from which electronsare escaped move to the emissive layer by the aid of the hole transportlayer. When the electrons and the holes run into each other at theorganic emissive layer, they are combined to form excitons having a highenergy potential. The excitons emit light while dropping to a low energylevel.

In order to facilitate the injection of electrons and improve theluminous efficiency, metals such as magnesium, magnesium-silver alloy,aluminum, lithium aluminum alloy and calcium have been generally used inOLED to form the cathode. However, when light is incident from theoutside of the OLED, some of the incident light is reflected on themetallic cathode since the metallic cathode has a high surfacereflectance. This internal reflection causes problems associated withthe degradations in contrast and visibility of the OLED.

Therefore, a circular polarizer including a linear polarizer and a ¼retardation plate has been used in the art to compensate for thisdegraded contrast of the OLED. However, the conventional circularpolarizer has a problem in that, although it is used to improve thecontrast, its transmittance is dropped by below 45% due to theabsorption of light by the circular polarizer, which leads to thesignificantly degraded brightness of the OLED.

In the conventional OLED devices, leading factors that degrade thebrightness of the OLED may be the total internal reflection caused bythe difference in refractive indexes of respective layers constitutingthe OLED, the polarization (light absorption) by the circular polarizer,etc. Light emitted from the emissive layer of the OLED may be reduced inbrightness by the reflection (i.e. total internal reflection) whilebeing passed through the hole transport layer, the hole injecting layer,the anode and the transparent substrate, and be finally reduced inbrightness by 10% or less while being passed through the circularpolarizer, which leads to low light emission efficiency.

In order to solve these problems regarding the degraded brightness, amethod of optically modifying a transmission path inwards the OLEDdevice or using special substances at respective layers has been underconsideration, but this alternative method has problems in that it isdifficult to be put to practical use due to the inefficient process andlow yield, and thus due to high manufacturing costs.

DISCLOSURE Technical Problem

The present invention is designed to solve some of the problems of theprior art, and therefore it is an object of the present invention toprovide a polarizer for organic light emitting diodes (OLED) havingimproved brightness, which is able to enhance a contrast ratio of theOLED and highly improve the brightness as well.

Technical Solution

According to an aspect of the present invention, there is provided apolarizer for organic light emitting diodes (OLED) having improvedbrightness, which includes a linear polarizer and a ¼ retardation plate,including a reflective polarizer film disposed between the linearpolarizer and the ¼ retardation plate and transmitting a polarized lighthorizontal to the transmission axis of the linear polarizer whilereflecting a polarized light vertical to the transmission axis of thelinear polarizer.

In this case, one or more adhesive layers may be present on the top andbottom of the reflective polarizer film. Here, at least one out of theadhesive layers may be formed of a color compensation adhesive fordecreasing the reflectance of external light.

Also, the color compensation adhesive may include an adhesive resin anda light absorbing agent. Here, the adhesive resin is selected from thegroup consisting of an acryl-based resin, a urethane-based resin, apolyisobutylene-based resin, a styrene-butadiene rubber (SBR)-basedresin, a rubber-based resin, a polyvinylether-based resin, anepoxy-based resin, a melamine-based resin, a polyester-based resin, aphenol-based resin, a silicon-based resin, or copolymers thereof, andthe light absorbing agent may include a black dye.

According to another aspect of the present invention, there is providedan OLED device including the polarizer for OLED as described above.

Advantageous Effects

The polarizer for OLED according to one exemplary embodiment of thepresent invention may be useful to enhance a contrast ratio of the OLEDand highly improve the brightness as well by disposing a reflectivepolarizer film between the linear polarizer and the ¼ retardation plate,the reflective polarizer film being able to selectively transmit alinearly polarized light.

Also, the polarizer for OLED according to one exemplary embodiment ofthe present invention may be useful to decrease the reflectance ofexternal light in order to improve the contrast ratio of the OLED byforming an adhesive layer on the top or bottom of the reflectivepolarizer film, the adhesive layer being formed of a color compensationadhesive and including a light absorbing agent.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a configuration of aconventional OLED panel.

FIG. 2 is a diagram illustrating the progress of light in theconventional OLED panel.

FIG. 3 is a diagram illustrating a configuration of an OLED panelaccording to one exemplary embodiment of the present invention.

FIG. 4 is a diagram illustrating the progress of light in the OLED panelaccording to one exemplary embodiment of the present invention.

BEST MODE

Hereinafter, exemplary embodiment of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram illustrating a configuration of aconventional organic light emitting diode (OLED) provided with acircular polarizer, and FIG. 2 is a diagram illustrating thetransmission of light in the conventional OLED panel provided with thecircular polarizer. As shown in FIG. 1, a circular polarizer 20consisting of a linear polarizer 24 and a ¼ retardation plate 22 isattached to an outer surface of a transparent substrate 11 of theconventional organic light emitting diode 10 in order to intercept thereflection of external light. In the conventional OLED panel thusconfigured, an externally incident light 110 is linearly polarized in acertain direction while being passed through a linear polarizer 24, andthe linearly polarized light 120 is converted into a circularlypolarized light 130 while being passed through a ¼ retardation plate 22.Meanwhile, a rotation direction of the circularly polarized light isreversed (a reversed polarized light 140) while this circularlypolarized light is being reflected on a metallic cathode 18, and thecircularly polarized light is also converted into a linearly polarizedlight 150 while penetrating a ¼ retardation plate 22. In this case,since a rotation direction of the reflected light is reversed, the lightafter the penetration of the ¼ retardation plate becomes the linearlypolarized light 150 having a polarization plane vertical to the originalpolarization plane. As a result, the light is not transmitted outwardssince the light absorbs into the linear polarizer 24 without thepenetration of the linear polarizer 24 (see FIG. 2).

Meanwhile, light emitted from an emissive layer of the OLED alsopenetrates the circular polarizer 20. Approximately 57% of the emittedlight absorbs into a circular polarizer 20 while penetrating thecircular polarizer 20, and thus the intensity of the penetrated lightaccounts for approximately 43% of the total intensity of light. Also,the light emitted from the emissive layer is passed through a holetransport layer, a hole injecting layer, an anode, a transparentsubstrate and the like prior to passing through the circular polarizer.In this procedure, the loss of light may occur due to the total internalreflection caused by the difference in refractive index of respectivelayers. Therefore, the finally penetrated light is substantially droppedto a very low level (i.e. 10% or less) of the light emitted from theemissive layer.

The present inventors have repeatedly made attempts to solve the aboveproblems, and found that the brightness of OLEDs may be highly improvedby applying a reflective polarizer film between the linear polarizer andthe ¼ retardation plate, wherein the reflective polarizer film is usedto selectively transmit a linearly polarized light while reflect theother light. Therefore, the present invention is completed on the basisof the above facts.

FIG. 3 is a schematic diagram illustrating a configuration of an OLEDpanel including a polarizer 30 according to one exemplary embodiment ofthe present invention.

As shown in FIG. 3, the polarizer 30 according to one exemplaryembodiment of the present invention includes a linear polarizer 34, areflective polarizer film 33 and a ¼ retardation plate 32. Here, one ormore adhesive layers may be formed among the linear polarizer 34, thereflective polarizer film 33 and the ¼ retardation plate 32. The linearpolarizer 34 and the ¼ retardation plate 32 used in the presentinvention are identical to those used in the conventional circularpolarizer. That is, the linear polarizer and the ¼ retardation plateinclude all of linear polarizers and ¼ retardation plates used in theart, respectively.

Next, the reflective polarizer film 33 is a film that is used totransmit a certain polarized light component and reflect the otherpolarized light components. Here, a wire grid polarizer, an LCDbrightness-enhancing film, a cholesteric liquid crystal film and thelike may be used as the reflective polarizer film 33. The reflectivepolarizer film, through which a polarized light component passed throughthe linear polarizer is penetrated and on which the other polarizedlight components are reflected, is used in the present invention, andmay be configured with the above-mentioned films.

When the reflective polarizer film 33 that selectively transmits orreflects the linearly polarized light is disposed between the linearpolarizer 34 and the ¼ retardation plate 32 according to the presentinvention, the progress of light is described referring to FIG. 4.

First, the progress of externally incident light in the OLED accordingto one exemplary embodiment of the present invention is described inmore detail. It is assumed that the total intensity of externallyincident light 210 is 100% as shown in FIG. 4, approximately 4% of theexternally incident light 210 is reflected on a surface of the linearpolarizer 34, and the other approximately 96% of the externally incidentlight 210 is passed through the linear polarizer 34. While theexternally incident light 210 is passed through the linear polarizer 34,approximately 53% of the externally incident light 210 absorbs into thelinear polarizer 34, and approximately 43% of the externally incidentlight 210 is passed through the linear polarizer 34. Therefore, theexternally incident light 210 is converted into a linearly polarizedlight 220. The linearly polarized light 220 is passed through thereflective polarizer film 33, and the linearly polarized light 230 isthen passed through the ¼ retardation plate 32. When linearly polarizedlight 230 is passed through the ¼ retardation plate 32, the linearlypolarized light 230 is converted into a circularly polarized light 240,and the circularly polarized light 240 is passed through respectivelayers of the OLED, and is then reflected on the cathode 18. In thiscase, a rotation direction of the circularly polarized light 240reflected on the cathode 18 is reversed (a reversed circularly polarizedlight 250). Then, the reversed circularly polarized light 250 isconverted into a linearly polarized light 260 while being passed throughthe ¼ retardation plate 32. In this case, the linearly polarized light260 becomes a linearly polarized light having a polarization planevertical to the original polarization plane. As a result, the linearlypolarized light is reflected on the reflective polarizer film 33 (areflected light 270) without the penetration of the reflective polarizerfilm 33. The reflected light 270 is converted into a circularlypolarized light 280 while being re-passed through the ¼ retardationplate 33. Then, the circularly polarized light 280 proceeds to thecathode 18, and a rotation direction of the circularly polarized light280 is reversed (a reversed circularly polarized light 290) while thecircularly polarized light 280 is being reflected on the cathode. Then,the reversed circularly polarized light 290 is passed through the ¼retardation plate 32. In this case, since the light 300 passed throughthe ¼ retardation plate 32 becomes a polarized light that may penetratethe reflective polarizer film, the polarized light is emitted outwardsafter sequentially passing through the reflective polarizer film 33 andthe linear polarizer 32. In this process, a significant proportion ofthe externally incident light absorbs and approximately 15% of theexternally incident light is finally emitted outwards.

Next, considering the light that is emitted from the emissive layer, thelight emitted from the emissive layer is passed through respectivelayers of the OLED, and then penetrates the ¼ retardation plate 32 andreaches the reflective polarizer film 33. Some of the light penetratesthe reflective polarizer film 33 (a penetrated light 410), and some ofthe light is reflected on the reflective polarizer film 33 (a reflectedlight 420). Then, the reflected light 420 penetrates the ¼ retardationplate 32 again. Since the light 410 penetrating the reflective polarizerfilm 33 may penetrate the linear polarizer 34, the penetrated light 410penetrates the linear polarizer 34, and is emitted outwards. On thecontrary, the light 420 reflected on the reflective polarizer film isconverted into a circularly polarized light 430 while being passedthrough the ¼ retardation plate. Then, the circularly polarized light430 is passed through the respective layers of the OLED, and is thenreflected on the metallic cathode 18. A rotation direction of thecircularly polarized light 430 is reversed (a reversed circularlypolarized light 440) while the circularly polarized light 430 is beingreflected on the metallic cathode 18. Then, the reversed circularlypolarized light 440 is re-passed through the respective layers of theOLED, and is then converted into a linearly polarized light 450 whilebeing passed through the ¼ retardation plate 32. In this case, since arotation direction of the circularly polarized light is reversed by thereflection, the linearly polarized light 450 formed after thepenetration of the ¼ retardation plate 32 is a polarized light that maypenetrate the reflective polarizer film. Therefore, the linearlypolarized light 450 is emitted outwards after sequentially passingthrough the reflective polarizer film and the linear polarizer. Sincethe light emitted from the emissive layer may be theoreticallycompletely emitted outwards through the above-mentioned procedures, itis possible to highly improve the brightness of the OLED.

When the polarizer according to one exemplary embodiment of the presentinvention is used for OLED as described above, the OLED may have ahighly improved light use efficiency and brightness, compared to theconventional OLEDs, since it may be used to highly improve the emissionefficiency of light emitted from the emissive layer and also to minimizethe reflection of the externally incipient light by inducing theabsorption of the externally incipient light into the linear polarizerto the maximum.

Meanwhile, the polarizer for OLED according to one exemplary embodimentof the present invention may include an adhesive layer formed of a colorcompensation adhesive for decreasing the reflectance of external light,the adhesive layer being formed in the top and bottom of the reflectivepolarizer film.

The color compensation adhesive is an adhesive having the effect ofabsorbing a predetermined intensity of light over the entire visible rayrange. Here, the color compensation adhesive may be formed by mixing anadhesive resin with a light absorbing agent for absorbing light ofvisible ray range.

In this case, adhesive resins widely used in the art, for exampleadhesive resins used in conventional adhesive sheets, adhesive films andthe like, may be used as the adhesive resin. For example, adhesiveresins that transmit light may be used as the adhesive resin, but thepresent invention is not particularly limited thereto. Examples of theadhesive resin that may be used herein include an acryl-based resin, aurethane-based resin, a polyisobutylene-based resin, styrene-butadienerubber (SBR)-based resin, a rubber-based resin, a polyvinylether-basedresin, an epoxy-based resin, a melamine-based resin, a polyester-basedresin, a phenol-based resin, a silicon-based resin, or copolymersthereof. An acryl-based adhesive is particularly preferred.

Meanwhile, a dye is widely used as the light absorbing agent. The dyethat may be used herein, for example, include a black dye, but thepresent invention is not particularly limited thereto. Therefore, dyesthat are dissolved in an organic solvent and show good transparency maybe used without any limitations. In addition, dyes, which are formed bymixing other dyes except for the black dye, may also be used instead ofthe black dye.

In this case, a content of the light absorbing agent in the adhesiveresin is preferably in a range of approximately 0.001 to 1 part byweight, based on 100 parts by weight of the adhesive resin. When thecontent of the light absorbing agent is less than 0.001 parts by weight,a color compensation property may be insufficiently presented, whereas,when the content of the light absorbing agent exceeds 1 part by weigh,the transmittance may be severely decreased.

Meanwhile, when compounds have an absorption wavelength at a visible rayrange, any of the compounds may be used without limitation as the lightabsorbing agent. Preferably, a single black dye or a mixture of coloreddyes may be used as the light absorbing agent. Here, the single blackdye functions to broadly absorb light of a wavelength close to thecenter, 550 nm, of the visible ray range, that is, a wavelength of 450to 650 nm, and more preferably 500 to 600 nm. More particularly, the useof the single black dye is desirable, and a dye mixture having a wideabsorption wavelength is also suitable. In particularly, unlike thenatural light, light having a specific wavelength range is emitted fromlight sources of R, G and B in the case of the OLED. In thisconsideration, it is more preferred to use dyes that do not absorb lightof the specific wavelength ranges emitted from the OLEDs but absorbvisible light having a wavelength range excluding the specificwavelength ranges. For example, when there is a dye that may moreeffectively absorb light of the visible wavelength range except for thespectic R, G and B wavelength range of the OLED, the dye may be used toeffectively enhance the transmission of internal light of the OLED, andto absorb a certain level of external light as well. Therefore, the dyesmay be very effectively used as a color compensation dye for adhesives.When it is assumed that a wavelength center of the R is 460 nm, awavelength center of the G is 530 nm and a wavelength center of the B is620 nm, a transmission spectrum, which may absorb light of relativelywide wavelength close to 500 and 580 nm wavelengths, which are out ofthe wavelength ranges of the R, G and B, is desirable to be used. Forthis purpose, a dye mixture, which is obtained by mixing a dye having anabsorption wavelength range close to 500 nm wavelength, for example, anabsorption wavelength range of 480 to 520 nm, and more preferably 490 to500 nm with a dye having an absorption wavelength range close to 580 nmwavelength, for example, an absorption wavelength range of 570 to 600nm, and more preferably 580 to 590 nm, may be used as the lightabsorbing agent.

When the color compensation adhesive including the above-mentioned dyesis used to form an adhesive layer, the entire transmittance of the OLEDmay be varied to control the transmission of internal light and thereflection of external light. Therefore, the color compensation adhesivemay contribute to the improvement in brightness of the internal lightand the maintenance of high visibility of the OLED.

As described above, when the adhesive layer is formed of a colorcompensation adhesive containing a light absorbing agent to absorblight, the polarizer for OLED may be useful to further enhance thecontrast by absorbing internal reflected light onto the adhesive layer.

MODE FOR INVENTION

Hereinafter, exemplary embodiment of the present invention will bedescribed in more detail.

Example 1

A conventional linear polarizer, which was obtained by stacking TACprotective films on upper and lower surfaces of a polyvinyl alcohol(PVA) film, was used as a base film. Then, the polarizer having improvedbrightness according to one exemplary embodiment of the presentinvention was prepared by sequentially attaching a reflective polarizerfilm (brightness-improving film, 3M) and a 140 nm-thick ¼ wavelengthretardation film (a COP elongation film having a normal dispersioncharacteristic) to a lower surface of the conventional linear polarizer.Here, the reflective polarizer film might transmit a certain linearlypolarized light at a transmittance level of 50%. Then, the preparedpolarizer was attached to an OLED panel (a 2-inch passive matrix-typeOLED panel).

Example 2

A conventional linear polarizer, which was obtained by stacking TACprotective films on upper and lower surfaces of a PVA film, was used asa base film. Then, a reflective polarizer film (brightness-improvingfilm, 3M) that might transmit a certain linearly polarized light at atransmittance level of 50% was attached to a lower surface of theconventional linear polarizer. Then, a color compensation adhesive whosetransmittance is adjusted to a level of 80% was prepared by adding ablack dye to an acryl-based adhesive (a mixing ratio=100:0.02), and thereflective polarizer film was coated with the color compensationadhesive to form a color compensation adhesive layer. Then, a 140nm-thick ¼ wavelength retardation film (a COP elongation film having anormal dispersion characteristic) was attached onto the colorcompensation adhesive layer to prepare a polarizer having improvedbrightness according to one exemplary embodiment of the presentinvention. Finally, the prepared polarizer was attached to an OLED panel(a 2.2-inch active matrix-type OLED panel).

Comparative Example 1

For comparison, an OLED panel (a 2.2-inch passive matrix-type OLEDpanel) to which the polarizer was not attached was used herein.

Comparative Example 2

A conventional circular polarizer consisting of a conventional linearpolarizer and a 140 nm-thick ¼ wavelength retardation film (a COPelongation film having a normal dispersion characteristic) was attachedto an OLED panel. Here, the conventional linear polarizer was obtainedby stacking TAC protective films on upper and lower surfaces of a PVAfilm.

Experimental Example 1

The OLED panels prepared in Examples 1 to 2 and Comparative examples 1and 2 were measured for brightness and reflectance.

The brightness was calculated after the intensity of light emittedduring the driving of each OLED panel was measured according to thewavelengths. Here, the brightness ratios of the OLEDs were listed in theparentheses, provided that it was assumed that the intensity of lightemitted from the OLED panel (Comparative example 1) to which thepolarizer was not attached was 100%.

Meanwhile, the reflectance was calculated by measuring the intensity oflight reflected on the front side of each OLED panel using a UVspectrometer. Here, the reflectance ratios of the OLEDs were listed inthe parentheses, provided that it was assumed that the reflectance ofthe OLED panel (Comparative example 1) to which the polarizer was notattached was 100%.

The measurement results are listed in the following Table 1.

TABLE 1 Brightness, cd/A (%) Reflectance, AU (%) Comparative example 119.8 (100)  69.40 (100)  Comparative example 2  8.6 (43.4) 3.76 (5.4)Example 1 15.7 (79.3) 26.03 (37.5) Example 2 12.8 (64.6) 10.81 (15.6)

As listed in Table 1, it was revealed that, when the conventionalcircular polarizer prepared in Comparative example 2 was used in theOLED panel, the OLED panel had a brightness of merely 43.4%, but, whenthe polarizers prepared in Examples 1 and 2 were used respectively inthe OLED panels, the OLED panels had highly improved rightness of 79.3%and 64.6%, respectively. Also, it was revealed that the OLED panel ofExample 2 having an adhesive layer formed therein had a good reflectanceup to a level of 15%. That is, it was seen that both the brightness andcontrast ratio of the OLED panel were highly improved when the polarizeraccording to one exemplary embodiment of the present invention was usedin the OLED panel.

1. A polarizer for organic light emitting diodes (OLED) having improvedbrightness, which comprises a linear polarizer, a ¼ retardation plate, areflective polarizer film disposed between the linear polarizer and the¼ retardation plate and transmitting a horizontally polarized light tothe transmission axis of the linear polarizer while reflecting avertically polarized light vertical to the transmission axis of thelinear polarizer, and one or more adhesive layers formed on an uppersurface and a lower surface of the reflective polarizer film, wherein atleast one of the one or more adhesive layers is formed of a colorcompensation adhesive for decreasing the reflectance of external light,and wherein the color compensation adhesive includes an adhesive resinand a light absorbing agent, and the light absorbing agent is a mixtureof dyes having an absorption wavelength of 490 to 500 nm and 580 to 590nm.
 2. The polarizer of claim 1, wherein the light absorbing agent ispresent in a content of 0.01 to 1 part by weight, based on 100 parts byweight of the adhesive resin.
 3. The polarizer of claim 1, wherein theadhesive resin is selected from the group consisting of an acryl-basedresin, a urethane-based resin, a polyisobutylene-based resin, astyrene-butadiene rubber (SBR)-based resin, a rubber-based resin, apolyvinylether-based resin, an epoxy-based resin, a melamine-basedresin, a polyester-based resin, a phenol-based resin, a silicon-basedresin, and copolymers thereof.
 4. An OLED device, comprising thepolarizer having improved brightness defined in claim
 1. 5. Thepolarizer of claim 1, wherein the light absorbing agent is a block dye.6. The polarizer of claim 3, wherein the black dye has an absorptionwavelength of 500 to 600 nm.